Nervous System

佚名

T he nervous system coordinates and organizes the functions of all body systems. This intricate network of interlocking receptors and transmitters is a dynamic system that controls and regulates every mental and physical function. It has three main divisions:

• Central nervous system (CNS): the brain and spinal cord (See Reviewing the central nervous system .)
• Peripheral nervous system: the motor and sensory nerves, which carry messages between the CNS and remote parts of the body (See Reviewing the peripheral nervous system .)
• Autonomic nervous system: actually part of the peripheral nervous system, regulates involuntary functions of the internal organs.

The fundamental unit that participates in all nervous system activity is the neuron, a highly specialized cell that receives and transmits electrochemical nerve impulses through delicate, threadlike fibers that extend from the central cell body. Axons carry impulses away from the cell body; dendrites carry impulses to it. Most neurons have several dendrites but only one axon.

• Sensory (or afferent ) neurons transmit impulses from receptors to the spinal cord or the brain.
• Motor (or efferent ) neurons transmit impulses from the CNS to regulate activity of muscles or glands.
• Interneurons , also known as connecting or association neurons, carry signals through complex pathways between sensory and motor neurons. Interneurons account for 99% of all the neurons in the nervous system.

From birth to death, the nervous system efficiently organizes and controls the smallest action, thought, or feeling; monitors communication and instinct for survival; and allows introspection, wonder, abstract thought, and self-awareness. Together, the CNS and peripheral nervous system keep a person alert, awake, oriented, and able to move about freely without discomfort and with all body systems working to maintain homeostasis.

Thus, any disorder affecting the nervous system can cause signs and symptoms in any and all body systems. Patients with nervous system disorders commonly have signs and symptoms that are elusive, subtle, and sometimes latent.

PATHOPHYSIOLOGIC CONCEPTS

Typically, disorders of the nervous system involve some alteration in arousal, cognition, movement, muscle tone, homeostatic mechanisms, or pain. Most disorders cause more than one alteration, and the close intercommunication between the CNS and peripheral nervous system means that one alteration may lead to another.

Arousal

Arousal refers to the level of consciousness, or state of awareness. A person who is aware of himself and the environment and can respond to the environment in specific ways is said to be fully conscious. Full consciousness requires that the reticular activating system, higher systems in the cerebral cortex, and thalamic connections are intact and functioning properly. Several mechanisms can alter arousal:

• direct destruction of the reticular activating system and its pathways
• destruction of the entire brainstem, either directly by invasion or indirectly by impairment of its blood supply
• compression of the reticular activating system by a disease process, either from direct pressure or compression as structures expand or herniate.

Those mechanisms may result from structural, metabolic, and psychogenic disturbances:

• Structural changes include infections, vascular problems, neoplasms, trauma, and developmental and degenerative conditions. They usually are identified by their location relative to the tentorial plate, the double fold of dura that supports the temporal and occipital lobes and separates the cerebral hemispheres from the brain stem and cerebellum. Those above the tentorial plate are called supratentorial , while those below are called infratentorial .
• Metabolic changes that affect the nervous system include hypoxia, electrolyte disturbances, hypoglycemia, drugs, and toxins, both endogenous and exogenous. Essentially any systemic disease can affect the nervous system.
• Psychogenic changes are commonly associated with mental and psychiatric illnesses. Ongoing research has linked neuroanatomy and neurophysiology of the CNS and supporting structures, including neurotransmitters, with certain psychiatric illnesses. For example, dysfunction of the limbic system has been associated with schizophrenia, depression, and anxiety disorders.

Decreased arousal may be a result of diffuse or localized dysfunction in supratentorial areas:

• Diffuse dysfunction reflects damage to the cerebral cortex or underlying subcortical white matter. Disease is the most common cause of diffuse dysfunction; other causes include neoplasms, closed head trauma with subsequent bleeding, and pus accumulation.
• Localized dysfunction reflects mechanical forces on the thalamus or hypothalamus. Masses (such as bleeding, infarction, emboli, and tumors) may directly impinge on the deep diencephalic structures or herniation may compress them.

Stages of altered arousal

An alteration in arousal usually begins with some interruption or disruption in the diencephalon. When this occurs, the patient shows evidence of dullness, confusion, lethargy, and stupor. Continued decreases in arousal result from midbrain dysfunction and are evidenced by a deepening of the stupor. Eventually, if the medulla and pons are affected, coma results.

A patient may move back and forth between stages or levels of arousal, depending on the cause of the altered arousal state, initiation of treatment, and response to the treatment. Typically, if the underlying problem is not or cannot be corrected, then the patient will progress through the various stages of decreased consciousness, termed rostral-caudal progression.

Six levels of altered arousal or consciousness have been identified. (See Stages of altered arousal .) Typically, five areas of neurologic function are evaluated to help identify the cause of altered arousal:

• level of consciousness (includes awareness and cognitive functioning, which reflect cerebral status)
• pattern of breathing (helps localize cause to cerebral hemisphere or brain stem)
• pupillary changes (reflects level of brainstem function; the brainstem areas that control arousal are anatomically next to the areas that control the pupils)
• eye movement and reflex responses (help identify the level of brainstem dysfunction and its mechanism, such as destruction or compression)
• motor responses (help identify the level, side, and severity of brain dysfunction).

Cognition

Cognition is the ability to be aware and to perceive, reason, judge, remember, and to use intuition. It reflects higher functioning of the cerebral cortex, including the frontal, parietal, and temporal lobes, and portions of the brainstem. Typically, an alteration in cognition results from direct destruction by ischemia and hypoxia, or from indirect destruction by compression or the effects of toxins and chemicals.

Altered cognition may manifest as agnosia, aphasia, or dysphasia:

• Agnosia is a defect in the ability to recognize the form or nature of objects. Usually, agnosia involves only one sense ― hearing, vision, or touch.
• Aphasia is loss of the ability to comprehend or produce language.
• Dysphasia is impairment to the ability to comprehend or use symbols in either verbal or written language, or to produce language.

Dysphasia typically arises from the left cerebral hemisphere, usually the frontotemporal region. However, different types of dysphasia occur, depending on the specific area of the brain involved. For example, a dysfunction in the posterioinferior frontal lobe (Broca's area) causes a motor dysphasia in which the patient cannot find the words to speak and has difficulty writing and repeating words. Dysfunction in the pathways connecting the primary auditory area to the auditory association areas in the middle third of the left superior temporal gyrus causes a form of dysphasia called word deafness: the patient has fluent speech, but comprehension of the spoken word and ability to repeat speech are impaired. Rather than words, the patient hears only noise that has no meaning, yet reading comprehension and writing ability are intact.

Dementia

Dementia is loss of more than one intellectual or cognitive function, which interferes with ability to function in daily life. The patient may experience a problem with orientation, general knowledge and information, vigilance (attentiveness, alertness, and watchfulness), recent memory, remote memory, concept formulation, abstraction (the ability to generalize about nonconcrete thoughts and ideas), reasoning, or language use.

The underlying mechanism is a defect in the neuronal circuitry of the brain. The extent of dysfunction reflects the total quantity of neurons lost and the area where this loss occurred. Processes that have been associated with dementia include:

• degeneration
• cerebrovascular disorders
• compression
• effects of toxins
• metabolic conditions
• biochemical imbalances
• demyelinization
• infection.

Three major types of dementia have been identified: amnestic, intentional, and cognitive. Each type affects a specific area of the brain, resulting in characteristic impairments:

• Amnestic dementia typically results from defective neuronal circuitry in the temporal lobe. Characteristically, the patient exhibits difficulty in naming things, loss of recent memory, and loss of language comprehension.
• Intentional dementia results from a defect in the frontal lobe. The patient is easily distracted and, although able to follow simple commands, can't carry out such sequential executive functions as planning, initiating, and regulating behavior or achieving specific goals. The patient may exhibit personality changes and a flat affect. Possibly appearing accident prone, he may lose motor function, as evidenced by a wide shuffling gait, small steps, muscle rigidity, abnormal reflexes, incontinence of bowel and bladder, and, possibly, total immobility.
• Cognitive dementia reflects dysfunctional neuronal circuitry in the cerebral cortex. Typically, the patient loses remote memory, language comprehension, and mathematical skills, and has difficulty with visual-spatial relationships.

Movement

Movement involves a complex array of activities controlled by the cerebral cortex, the pyramidal system, the extrapyramidal system, and the motor units (the axon of the lower motor neuron from the anterior horn cell of the spinal cord and the muscles innervated by it). A problem in any one of these areas can affect movement. (See Reviewing motor impulse transmission .)

For movement to occur, the muscles must change their state from one of contraction to relaxation or vice versa. A change in muscle innervation anywhere along the motor pathway will affect movement. Certain neurotransmitters, such as dopamine, play a role in altered movement.

Alterations in movement typically include excessive movement ( hyperkinesia ) or decreased movement ( hypokinesia ). Hyperkinesia is a broad category that includes many different types of abnormal movements. Each type of hyperkinesia is associated with a specific underlying pathophysiologic mechanism affecting the brain or motor pathway. (See Types of hyperkinesia .) Hypokinesia usually involves loss of voluntary control, even though peripheral nerve and muscle functions are intact. The types of hypokinesia include paresis, akinesia, bradykinesia, and loss of associated movement.